1. Field of the Invention
The present invention relates to a composition of a piezoelectric (dielectric) material which is widely applied to fields such as actuators, sensors, and microwave resonators.
2. Related Background Art
A piezoelectric material is a dielectric material, in other words, a piezoelectric material is distorted due to an electric field applied externally (conversion of electric energy to mechanical energy) and it generates a charge on the surface thereof due to an external stress (conversion of mechanical energy to electric energy), and it has been widely applied to fields such as actuators and sensors. For example, a piezoelectric material is advantageously applied to minute positioning and used for fine-controlling optical systems because the amount of strain thereof is substantially in proportion to the applied electric field when estimated in the 10.sup.-10 m/V order (in the case of PZT). Further, to the contrary, since the amount of charge is in proportion to the applied stress or the amount of strain caused by the stress, the piezoelectric material is also used for sensors detecting a minute stress or strain. In addition, because of the excellent response of the piezoelectric material, it is also possible to cause resonance of the piezoelectric material by exciting the material per se or an elastic body connecting thereto by applying an AC electric field.
An ultrasonic motor is an example of the above and utilized for an auto-focusing mechanism of cameras, etc. Furthermore, a piezoelectric gyro and an active damper are examples utilizing both characteristics of the piezoelectric material such that it is distorted in proportion to the applied electric field and generates a charge in proportion to the applied stress and the amount of strain thereof. In the former, an elastic body is excited to resonant by applying an AC electric field to a piezoelectric material; and when an angular speed due to a rotary movement is externally added thereto, the speed is quantified such that the Coriolis force generated therein is converted again to an electric signal by the piezoelectric material. Meanwhile, in the latter, externally applied vibration is quantified by converting it to an electric signal using the piezoelectric material; and the vibration is compulsorily canceled by applying an AC electric field having an opposite phase.
A majority of piezoelectric materials practically used today belong to a solid solution system (PZT system) composed of PbZrO.sub.3 (PZ)-PbTiO.sub.3 (PT). This is because a material having excellent piezoelectric characteristics can be obtained by using a composition near the morphotropic phase boundary (M.P.B.) between PZ of the trigonal system and PT of the tetragonal system. By adding a variety of sub-components and additives to a composition of M.P.B., a wide range of compositions have been developed to meet various kinds of demand. Examples of these compositions are those which are suitably used for actuators and the like for positioning because of a small quality factor (Q.sub.m) and a large piezoelectric constant (d.sub.ij) and those which are suitably used for an ultrasonic wave generating element of an ultrasonic motor or the like because of a large quality factor (Q.sub.m) and a small piezoelectric constant (d.sub.ij).
Meanwhile, an electrostrictive material has similar characteristics to a piezoelectric material. In the case of electrostriction, the amount of strain is in proportion to the square of the applied electric field. Further, an electrostrictive material is advantageous over a piezoelectric material in that the strain induced by electric field is not accompanied with hysteresis because it utilizes the paraelectric phase of a relaxer ferroelectric material. Meanwhile, in the case of a piezoelectric material, the strain induced by electric field is accompanied with hysteresis, since a ferroelectric material is used for a piezoelectric material to overcome some problems, such as polarization treatment required when the material is used as ceramic.
A majority of electrostrictive materials practically used today are complex oxides the basic composition of which is Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 (PMN). This is because PMN fulfills both of the following requirements: a material is required to have as largest dielectric constant as possible because the electrostrictive constant thereof is in proportion to the square of the dielectric constant; and a material is required to have a Curie temperature (T.sub.c) slightly lower than room temperature so as to stay in the paraelectric phase at a temperature, at which the material is used, for canceling hysteresis accompanied with strain induced by electric field.
A majority of other piezoelectric and electrostrictive materials practically used today have a solid solution composition essentially consisting of a lead-type perovskite composition, such as PZT and PMN.
However, those lead types piezoelectric (electrostrictive) materials contain a large amount of lead oxide (PbO), which is highly volatile even at a low temperature, as a main component. For example, approximately two thirds by weight of PZT and PMN are lead oxide. Heat treatments, such as sintering and melting, are essential for manufacturing ceramic or single crystals from those piezoelectric (electrostrictive) materials, thus an extremely large amount of lead oxide volatilizes and diffuses into the atmosphere, when considering it from industrial point of view. Consequently, a demand for mitigating pollution, that is, non-leaded materials, will be inevitably generated in the future corresponding to increases in the applied fields and the amount of usage of piezoelectric (electrostrictive) ceramic and single crystals. Therefore, non-leaded or low-leaded materials having excellent piezoelectric (electrostrictive) characteristics are in demand from an ecological point of view and for preventing pollution.